list prerequisite software in README

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 # challenge-bot
 
+## prerequisites
+
+install the following software:
+
+[openSCAD](http://www.openscad.org/downloads)
+
+[fritzing](http://fritzing.org/download/)
+
+[arduino ide](http://arduino.cc/en/Main/Software)
+
 ## quickstart
 
 [openSCAD tutorial](http://xib.me/s2ry)